Cold-working process



Oct. 6, 1970 Filed Oct. 20, 1967 MITSUO TOM IOKA ETAL COLD-WORKINGPROCESS I 3 Sheets-Sheet 1 fl\ lVENT|VE PROCESS WIRE ROD EIRsT DRAWNWIRE HEAT TREATMENT FINISHE DRAWN WIRE COLD HEADING THREADING FINISHEDBOLT Fl N SHED CONVENTIONAL PROCESS BOLT MITSUO TOMIOKA KOIC HI URAKAWAINVENTOR 1970 MITSUO TOMIOKA ETAL 3,532,560

COLD-WORKING PROCESS Filed Oct. 20, 1967 3 Sheefs-Sheet 2 l o o\REDUCTION OF AREA BREAKING %IN COLD PRE S a,

4'0 5'0 66 7'0 ov/Q) REDUCTION OF AREA%M'1Ts-U0 TOMIOKA 'W- KOICHIURAKAWA Oct. 6, 1970- Filed Oct. 20, 19s? MITSUO TOMIOKA ETAL 3,532,560

COLD-WORKING PROCESS s she t sh et 5 MITSUO TOMIOKA KOICHI URA KAWA,INVENTORS United States Patent Oflice 3,532,560 Patented Oct. 6, 19703,532,560 COLD-WORKING PROCESS Mitsuo Tomioka, Nishinomiya-shi, andKoichi Urakawa, Kobe-shi, Japan, assignors to Kobe Steel Works, Ltd.Continuation-impart of application Ser. No. 358,317, Apr. 8, 1964. Thisapplication Oct. 20, 1967, Ser. No. 677,507 Claims priority, applicationJapan, Apr. 18, 1963, 38/ 20,395 Int. Cl. C21d 9/52, 1/18 US. Cl.148-12.4 3 Claims ABSTRACT OF THE DISCLOSURE CROSS-REFERENCE TO RELATEDAPPLICATION This application is a continuation-in-part of applicationSer. No. 358,317 filed Apr. 8, 1964 now abandoned.

BACKGROUND OF THE INVENTION The present invention relates to a processfor producing cold-shaped or forged products having excellent mechanicalproperties and high fatigue limits from a length of a continuouselongated shaped steel article such as steel wire, rods, bars and thelike (hereafter it should be recognized that although the discussion isdirected to steel wire, other continuous elongate shapes could be used)which has been previously tempered under particular conditions.

DISCUSSION OF THE PRIOR ART Bolts, studs, pins and other machine partswhich are required to have high tensile strength (in excess of 70kg./mm. are usually produced by a process in which carbon steelcontaining carbon in excess of 0.3% by weight or steel alloy containingan alloy element selected from the group consisting of chromium, nickeland molybdenum is processed by cold-forging, hot-forging or cutting thesteel material and then subjecting the thus obtained intermediateproduct to a quality adjusting step such as hardening or tempering so asto impart predetermined mechanical properties to the final product.Alternatively, according to another conventional process, a length ofpreviously tempered steel material is cut to the desired size and shapeto provide a final product or machine part having predeterminedmechanical properties.

The above enumerated machine parts may also be produced by aconventional cold-shaping process which usually comprises selecting alength of coiled steel wire (this material will be merely referred to assteel wire hereinafter) as the starting material to be processed andthen cold shaping the steel Wire in a cold shaping machine in acontinuous operation. However, if the steel wire material containscarbon in excess of 0.3% by weight or has an excessively high degree ofhardness, the steel wire may break or fissure during the cold shapingoperation or the thus obtained product may have insufficient tensilestrength. Thus, in an effort to eliminate these disadvantages thecold-shaping processes of the prior art frequently use the so-calledspherically annealed steel wire as the starting material to becold-shaped. The annealed steel wire is then cold shaped and subjectedto a further tempering treatment so as to obtain a machine part orproduct having a high tensile strength and predetermined mechanicalproperties. Therefore, in order for conventional steel wire to besuitably employed as the starting material to be cold-shaped into amachine part, the steel Wire must be first subjected to an annealing orspherical annealing treatment in order to reduce the hardness of such asteel wire to a level suitable for the cold shaping operation. However,since such annealing treatments are carried out at a high temperatureranging from 700 to 900 C. for a rather long period of time covering 5to 10 hours, the steel wire is liable to form a deoxidized layer orlayers during the annealing treatment. Further, since the temperingtreatment is the last stage in a series of stages involved in the coldshaping of a machine part, the tempering treatment has to be carried outin a non-oxidizing atmosphere furnace or salt bath soaking pit;otherwise it is quite difficult to obtain a product having satisfactoryquality.

In addition to the above disadvantages, there is the additionaldisadvantage that different lots of the products will inevitably havedifferent qualities and dimensions from each other because they aresubjected to the heat treatment in lots, and, therefore, the productsmay fail to show their expected performance and may fail after they areincorporated into a machine.

Another prior art process is the so-called patenting process which hasbeen widely used for the purpose of continuous heat-treating of steelwire. However, the patenting process is applicable to only theproduction of high tensile strength steel wires such as hard steelwires, piano wires and spring steel Wires and this process has beendeveloped to cold-stretch high carbon steel wires having carbon contentsin excess of 0.6% by weight. To put it more concretely, the patentingprocess comprises the steps of heating steel wire to a temperature aboutits A3 transition point and quenching the steel wire in a lead bathsoaking pit heated to 400 to 600 C. so as to temper the steel material.Since the patenting treatment primarily has been developed as apre-treatment step in the coldstretching operation of hard steel wireshaving high carbon contents such as piano wires for example, thepatenting treatment is not applicable as a pre-treatment process in thecold-forging of steel wires having carbon contents less than 0.6% byweight for producing machine parts. That is, it is possible to obtainfine grain structures for high carbon steel wires having carbon contentabove 0.6% by weight, but in case of carbon steel wires having a carboncontent of less than 0.6% by weight, the wires treated by the patentingprocess will have uneven grain structures due to insuflicient quenching.And when steel alloys containing manganese, nickel and chrome in a rangeof 1 to 3% respectively are treated by the patenting process, suchtreated wires will have uneven grain structures wherein bath decomposedand undecomposed portions co-exist, and thus, by the patentingtreatment, uniform fine grain sorbite structures for steel wires cannotbe obtained.

In short, by the conventional patenting process, it is impossible toobtain uniform fine grain sorbite structures for steel wires havingcarbon contents or 0.2 to 0.6% by weight to such degree that the steelwires may be cold-forged into final products or machine parts withoutthe necessity for further heat treatment after production thereof.

The novel process of our invention makes it possible so obtain uniformfine grain sorbite structures for steel wires and this process can beclearly distinguished from the conventional patenting treatment process.

In addition to the patenting treatment process, many other processeshave been so far proposed for heat treating steel wires continuously,but unfortunately, no technical ideas exist which advance such priorarts to the production of machine parts such as bolts and pins throughcold-forging. The present invention is important in that the samecontinuous steel wire heat treatment technology is utilized for theproduction of cold-forged or shaped products or machine parts by the useof tempered steel wires.

SUMMARY OF THE INVENTION In this sense, it can be said that the presentinvention does not represent a merely mechanical combination of heattreatment and soldshaping steps for steel wires, but rather is a novelprocess which has been developed on the basis of findings of theworkabilities of materials to be cold-forged or shaped.

After a thorough study of workabilities of materials to be cold-shaped,we have found that the efiiciency of the cold-shaping operation hasalmost nothing to do with the degree of hardness of the particular steelwire to be cold-shaped, but rather greatly depends upon the mechanicalproperties of the particular steel wire employed, especially thedrawability and microstructure of the steel wire. Based on the abovediscovery we have provided a novel cold-forging process which includes atempering step and which eliminates the above-mentioned disadvantagesinherent in the conventional cold-shaping processes. According to thepresent invention, there is provided a improved cold-forging process forproducing cold-shaped products which comprises the steps of rapidlyheating a running length of carbon steel wire or steel alloy wire to atemperature above the A3 transition point of the steel material whilecontinuously reeling out of a roll of coiled wire; quenching said heatedsteel wire by means of cooling agent such as water or oil so as toharden the wire; rapidly heating said quenched steel wire to atemperature 300 to 700 C.; cooling said heated wire to room temperatureby means of cooling agent such as water or compressed air so as totemper the wire whereby a length of tempered wire having a uniform finegrain sorbite structure is obtained; and cold-forging said temperedsteel wire into products having prescribed shape and dimention by acold-forging machine. The steel wires suitably employed as the startingmaterial in carrying out the novel process by the present inventioninclude carbon steel wire containing carbon in an amount of 0.2 to 0.6%by weight and alloy steel wire containing one or more of silicon,manganese, nickel, chromium, molybdenum, vanadium and boron in an amountless than 3% 'by Weight respectively in addition to carbon in the aboveprescribed content range. That the amount of carbon be within theaforementioned range is particularly critical; this criticality will beexplained in greater detail below in the preferred embodiment.

DESCRIPTION OF THE DRAWINGS In respect to the drawings:

FIG. 1 shows a block flow sheet for a conventional cold-shaping processand for the process of our invention;

FIG. 2 graphically shows the relationship between micro-structures andpercentage of break or crack occurrence for various types of steelwires;

FIG. 3 graphically shows the relationship between critical coldcompressive amounts and percent reductions in area on variouscold-forged steel wires; and

FIG. 4 is a graph of load-number cycle curves for bolts produced by ourprocess and bolts prepared by conventional processes-as discussed inExample I.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Considering the drawings ingreater detail, FIG. 1 shows a flow sheet comparison between aconventional prior art process and the novel process of our invention.As is readily apparent on the face of this figure, our process, as wellas producing a superior product, results in the elimination of severalcostly and time-consuming steps required in the prior art process.Further advantages of our process and the products produced thereby areenumerated as follows:

(1) Since, according to our process, the steel wire used as the startingmaterial is continuously subjected to the tempering treatment in itswire form as it was produced, the thus tempered steel material hasuniform mechanical properties and microstructure throughout its length,and, accordingly, disparity in quality of products processed from such atempered material is quite rare.

(2) Since products or cold-forged parts processed from the steel wiretempered by our process do not require any further heat treatment afterproduction thereof, accuracy in shape and dimension of such products canbe maintained and, especially, the same is true in the case of smalldiameter and long products.

(3) Because the effect of residual compression stress on the surfaces ofcold-forged products, which are characteristic of such products, theobtained products have various excellent mechanical properties,especially an excellent resistance to repetitive fatigue.

(4) Since our process eliminates the time-consuming annealing orspherically annealing treatment which is necessary in the conventionalprocesses for preparing steel wires, there will be no possibility forthe formation of deoxidized layers as in the conventional sphericalannealing treatment; that is, this opportunity for decrease in surfacestrength and decrease in resistance to fatigue can be prevented.

(5) By preparing cold-forged products from tempered steel wire inaccordance with our process, as compared with the conventionalprocesses, the actual production process can be greatly simplified,resulting in a reduction in production costs as well as improvement inquality.

FIGS. 2 and 3 are particularly useful in considering the workability ofmaterials to be cold-forged, especially since as to the workabilities ofmaterials to be coldforged, there has been no established definition upto date. However, through our strenuous studies of the workabilities ofvarious materials we have concluded that the cold workabilities of steelmaterials are essentially determined depending upon the chemicalcompositions and heat treatment processes, and expressed in the terms ofmechanical properties and microstructures of such materials. Up to now,it Was considered that the workability of materials having high hardnessand high tensile strength was poor. However, after our studies andexperiments, we are convinced that even some steel materials having hightensile strengths can be properly cold-forged without difficultyprovided that they satisfy certain specific requirements. This pointwill be explained in detail referring to FIGS. 2 and 3 of theaccompanying drawings.

FIG. 2 graphically shows relations between microstructures of varioustypes of steel wires produced under different heat treatment conditions,such as 835C (AISI 1035), S450 (AISI 1045), SCrZ (A181 5130), SCrR (AISI5140), SCM3 (AISI4135), SCr21 (AISI 5130) and the subsequentparenthetical terms indicating the standard U.S. nomenclature for thesame steel wire; reductions of area of these wires determined throughtension tests (horizontal), and percentage of break or crack occurrenceduring cold-forging with 80% of coldcompression ratio (vertical). Inthis figure, Curve A designates an annealed coarse grain structure steelWire, Curve B designates an annealed fine grain structure steel wire,Curve C designates a spherodized fine grain structure steel wire, andCurve D designates a tempered fine grain structure steel wire.

From the graph of this figure, it is clear that as the grain structureschange from A to D the frequency of coldforging breaking or crackingbecomes less. In order to completely eliminate cold-forging breaks, thereduction in area of the steel wire must be above 70% in A wire andabove 65% in B wire, respectively, whereas C and D wires show nocold-forging breaks even when the reduction in area is 50%. Thus, it isnoted that the cold-workabilities of steel wires are greatly influencedby the mechanical properties and microstructures of the steel materials.

FIG. 3 shows graphically the relations between critical cold compressiveamounts (the point at which a break takes place in steel wires) in coldcompression tests and reductions of area percent in tension testsperformed on various cold-forged steel wires which were coldstretchedunder different drawing ratios. From the graph of FIG. 3, it is clearthat even the drawing ratio of coldforged steel wires amounts to suchgreat degrees as to Further, if the chemical compositions of steelmaterials and heat treatment conditions for such materials are soselected that the reductions of area in such materials may be great, thebreaking point of such materials can be raised, that is, the coldworkability of such materials can be improved.

The novel invention utilizes these excellent cold workabilities oftempered steel wires as shown in the graphs of FIGS. 2 and 3.Especially, since D steel wire has a fine grain sorbite microstructure,it is noted that this D wire has a high tensile strength and excellentworkability. Therefore, it is mandatory that in order to industriallyutilize such properties of these steel wires in producing cold-forgedproducts from such steel wires, the chemical compositions of such steelwires and the treatment conditions for such materials should be properlycombined depending upon the properties, strength and shape called for inthe cold-forged products prepared from such materials. This novelcombination of chemical compositions and heat treatment conditions canbe achieved only by the present invention.

A specific and preferred embodiment of our invention will now bedescribed. In the first step of our process, a plain carbon steelcontaining about 0.2 to 0.6% by weight carbon or an alloy steel alsocontaining this percent of carbon and one or more of an alloyingcomponent such as silicon, manganese, nickel, chromium, molybdenum,vanadium, and boron, and preferably containing not more than 3% byweight of any given alloying metal, is selected.

The reason for which the amount of carbon to be contained in carbonsteel wire as the starting material is specified as the range 0.2 to0.6% by weight is that if the carbon content of the steel material isless than 0.2% by weight, the carbon content during the temperingtreatment is insufficient to impart the prescribed mechanical strength,which will be called for by the desired product or machine part; and, ifthe carbon content of the steel wire is in excess of 0.6% by weight, acold-forged product prepared from such steel material will not possesssufficient tensile strength to meet requirements for the desiredcold-forged product. Where alloy steel is employed as the startingmaterial to be cold-forged, the reason that that content for each of thealloying elements is specified as less than 3% by weight is that if oneor more of these elements in the prescribed amount are properlyincorporated into the base steel material, a product having excellentmechanical properties can be easily obtained through the temperingtreatment. Although it is possible to use these alloying elements inexcess of the prescribed amount, there may be no notable improvements inmechanical properties of the product over those of a product preparedfrom alloy steel wire containing the allowing elements in the prescribedamounts respectively, and, accordingly, the use of these alloyingelements which are exepnsive in excess of 3% respectively isuneconomical. The incorporation of from 0.0007 to 0.003% by weight ofboron into the steel will improve the heat treatment capability of thealloy steel wire and, accordingly, economy of the other alloying elementor elements which are greater in amount than that of the boron additive.

Steel wire or alloy steel having the above-mentioned chemicalcompositions may be used as the starting material for our process in theform of unstretched wire as it is produced or these steel materials maybe stretched prior to being subjected to our tempering treatment. Whensuch stretcihng is used the amount of stretching is usually such as toeffect a 13% reduction in cross-sectional area. In any case, through thetempering treatment of such steel materials, a tempered intermediateproduct having mechanical properties and uniform fine grain sorbitestructure suitable for continuous cold-forging will be obtained.

In the next step of our process, the selected plain carbon steel wire oralloy steel wire having the required composition is rapidly heated to atemperature above the A3 transition point of the steel material of thesteel wire.

This heating may be effected by continuously passing the steel rod,preferably at a rate of 2 to 6 in. per minute, through a suitableconventional high temperature heating means, such as for example a highfrequency heating furnace, flame heating furnace, electric resistanceheating furnace, electric furnace, heavy oil furnace, salt bath soakingpit or lead bath soaking pit, so that the steel wire may be maintainedat a temperature range of 850 to 950 C. for l to 3 minutes. The purposeof the rapid heating of the steel wire is to obtain a uniform austenitestructure of the steel wire. Also, heating of the steel wire to atemperature below 850 C. is insufficient to obtain a uniform austenitestructure of the steel wire, and, accordingly, the insufficiently heatedsteel wire is difiicult to harden uniformly. Alternatively, if the steelwire is heated to a temperature above 950 C., the steel wire will beoverheated, resulting in deoxidation layer and/or rough surfaceformation which leads to a poor surface quality or coarsely hardenedstructure. Therefore, the rapid heating of the steel wire within thetemperature range of 850 to 950 C. for a time space of 1 to 3 minutes iscritical in order that the steel wire may be properly treated in theheat treatment step of the present process.

After having been rapidly heated to the temperature range of 850 to 950C., the heated steel wire is passed to a quenching device, which ispreferably directly connected to the preceding heating surface, wherethe heated steel wire is quenched by means of any suitable coolingagent, such as water or oil, until the innermost portion of the interiorstructure of the steel wire is cooled to a temperature below about 200C. This provides the steel wire with a uniformly hardened structurethroughout its mass. The type of cooling agent may be suitably selecteddepending upon the chemical compositon and diameter of the steel wire tobe processed. When the diameter of the steel wire is smaller than 11mm., oil is employed as the quenching or hardening agent whereas steelwire having a diameter greater than 11 mm. is quenched or hardened bymeans of water. In the quenching or hardening of the steel wire, it isnecessary that the steel material be quenched until the interiorstructure of the steel material will also have a uniformly quenchedstructure as well as the exterior thereof. However, when the quenchingis effected at an extremely rapid rate, breaks may occur in the steelwire. Therefore, it is preferable that the steel wire be quenched untilthe innermost portion of the interior structure of the steel material iscooled to 200 C., and thereafter the steel material is gradually cooled.

In order to determine whether the steel wire has been quenched touniformly hardened structure or not, a microscopic test on the structureof the thus treated steel wire is carried out or a tension test on thetensile strengh of the thus treated steel wire is carried out. In theprocess of the present invention, after the steel wire having theabove-mentioned chemical composition has been subjected to the quenchingor hardening treatment in the manner described above, it has been foundthat the treated steel wire has a tensile strength above 140 kg./ mm. Solong as the tensile strength is not below the above value, uniformquenching or hardening is always attained.

The steel wire, now having a uniformly hardened structure, is thenpassed through a second heating furnace which is preferably connected tothe above-mentioned quenching device and is maintained at 300 to 700 C-where the steel wire is evenly heated for a period of 2 to 10 minutes.The heated steel wire is then passed to a second cooling device which ispreferably connected to the second heating surface where the steelmaterial is cooled or tempered to room temperature by means of water orcompressed air. The reason for specifying a heating temperature fortempering in the range mentioned above is that such a temperature rangeis necessary to impart mechanical properties suitable for cold-forging ithe specific type of steel wires to be processed by the process of thepresent invention, and especially for obtaining a desired area reductionvalue for the steel wires in a tension test and a fine grain sorbitemicro-structure suitable for cold-forging the steel wires.

If the heating temperature for tempering is below 300 C., decompositionof the martensite structure of the steel Wire in the hardened state intothe fine grain sorbite structure thereof is insufficient and, althoughthe tempered steel wire will have a high strength, its tenacity will betoo low, and the steel wire will be unfit for cold-forging. Conversely,if the tempering heating temperature is in excess of 700 C., thedecomposition of the martensite structure of the steel wire into thesorbite structure is too rapid, resulting in a coarse grain sorbitestructure or a 2 pearlite structure which reduces the tensile strengthof the steel wire to a value below 70 kg./mm. Therefore, a temperingheating temperature in excess of 700 C. is objectionable where it isdesired to produce a tempered steel suitable for producing a cold-forgedproduct having a tensile strength above 70 kg./mm. Accordingly, theheating temperature for tempering is an important factor and the heatingor tempering temperature should be suitably selected within thespecified temperature range of 300 to 700 C. depending upon the tesilestrength desired in the final product, the diameter of the steel wireemployed as the starting material, and the processing rate of the steelwire material. The cooling of the steel wire material from the temperingtemperature to room temperature need not be rapid.

By the above heat treatment or tempering treatment, an intermediatesteel product having a uniform fine grain sorbite structure and atensile strength of 70120 kg./ mm. can be easily produced in a shortspace of time. The intermediate or tempered wire product is thensubjected to a surface treatment such as pickling, lime and phosphoricacid film formation in the conventional manner so as to obtain aso-called tempered steel wire which may then be suitably cold-forged byconventional coldforging machine into desired final products or machineparts.

In some cases, prior to the cold-forging operation, the tempered steelwire is stretched by an amount less than reduction in area (usuallyabout 13%) so as to ad just its diameter and tensile strength. The thusobtained tempered steel wire of uniform fine grain sorbite structure isthen continuously cold-forged through a cold-forging or shaping machineso as to produce machine parts having a tensile strength of 70 to 120kg./mm. a high hardness and a high tenacity. Since the steps in theprocess of the present invention are carried out in succession,production efiiciency is quite high; further, the shaped or forgedproducts do not require any heat treatment after production, andproducts of precise dimensions can be easily obtained.

8 Our invention is further illustrated by the following examples but isnot limited thereto.

Example I This example illustrates our inventive process using a steelwire as the starting material having the following chemical composition:

Percent C 0.30 Mn 1.60 Si 0.30 P 0.030 S 0.30

A length of coiled steel wire having the above chemical composition (11mm. in diameter and 350 kgs. in weight per roll) was first coldstretched to a diameter of 10.2 mm. and then passed through a Muflietype heavy oil furnace at a rate of 5 m./min. so as to heat the steelwire to a temperature below about 900 C. The thus heated wire was thenquenched in an oil bath so as to reduce the temperature of the wire to atemperature below 200 C. The quenched wire was then tempered by beingpassed through a lead bath maintained at 600 C. at the same rate as thatat which the wire was passing through the heavy oil furnace. After thelead bath tempering treatment the diameter of the wire was reduced to9.45 mm. by cold finish stretching to obtain a length of tempered steelwire which was suitable for producing bolts of diameter by cold forging.

The thus treated steel had a uniform fine grain sorbite microstructureand the following mechanical properties:

Yielding point82 kg./mm. Tensile strength87 kg./mm. Elongation20%Reduction of area66%.

Next, this tempered steel wire was cold-shaped or forged by a coldshaping machine so as to form hexagon headed bolts of /8 diameter. Theshaping machine produced six hexagon headed bolts per minute. Theobtained bolts were further subjected to a screw-thread forming stepwhile they were turned round to produce complete /s" x mm. hexagonheaded bolts.

The bolts were tested in accordance with the 118 bolt test procedure todetermine their mechanical properties and the determined mechanicalproperties were compared with those of comparable bolts produced by theconventional processes. The bolt test procedure is the same as the SAEStandard for Mechanical and Quality Requirements for ThreadedFasteners--SAE 1429C.

The results of the comparison are given below:

The 30 wedge test refers to a tension test which was performed byplacing a 30 angle wedge between the bottom surface of the head of thebolt and the supporting surface of a supporting seat. Further, inaccordance with the standard DIN procedure (i.e., DIN 267Bolt Screws,Nuts and Similar Threaded and Formed Parts Technical Condition ofDelivery), bolt head impact tests were performed on the above threetypes of bolts and good results were obtained on all of these bolts.

In addition, bolts produced by our process in the above Example I werecompared with bolts made of 545C (AISI 1045) and SCrZ (AISI 5130), bythe conventional process, by using Baldwin-type universal fatiguetesting machine. Data was then obtained from which load-number of cyclecurves were prepared. The load was selected in the range of 2400-3600kg., the cycle being in the range of 10 -5 X Curves a, b, and c wereobtained as shown in FIG. 4. Curve a shows the results obtained from thebolts produced in accordance with the present invention, curve b beingthe results of conventional bolts made of AISI 1045, while curve 0 showsthe results of conven tional bolts made of A151 5130. The load at thecycle of 2x10 is shown in the following table. The ratio of the loadwith respect to the yield strength in the static tension test of thebolts is also shown in the table.

TABLE Load, kg. Ratio (a) Bolts of the present invention 3,290 Y.P. 72%.(b) Bolts of AlSI 1045 2,860 Y.I. 07%. (c) Bolts of A181 5130 3,000 Y.P.t8%.

EXAMPLE II This example illustrates our invention using a steel wire asthe starting material having the following chemical composition:

Percent A length of coiled steel wire having the above chemicalcomposition (11 mm. in diameter and 360 kg. in weight per roll), from aroll of such a wire was first coldstretched to a diameter of 8.5 mm. andthen passed through a lead bath soaking pit maintained at 880 C., whichused light oil as its fuel, at a rate of 2 m./rnin. so as to heat thesteel wire to 880 C. in three minutes. The wire was then cooled to 200C. by being passed through an oil bath whereby the steel wire washardened to have a uniformly hardened structure. The tensile strength ofthe hardened steel wire was above 150 kg./mm. The hardened steel wirewas then passed through a tempering lead bath soaking pit maintained at630 C., at the same rate as that at which the steel wire was passedthrough the first lead bath soaking pit. The steel wire maintained inthis bath at a temperature of the steel wire at 630 C. for five minutes.The steel wire was then cooled to room temperature by means ofcompressed air. The abovementioned series of treatments wereconsecutively performed throughout the entire length of the steel wireand the resulting steel wire had a uniform fine grain sorbite structureand a tensile strength of 85 kg./mm. The thus treated steel wire wassubjected to pickling and phosphate film forming treatment and thencold-stretched to reduce its diameter to 7.93 mm. so as to obtain alength of tempered steel wire to be suitably employed to produce I IS 8mm. diameter bolts. The tempered steel wire had a uniform fine grainsorbite structure and the following mechanical properties:

Yield point87 kg./mm. Tensile strength91 kg./mm. Elongation% Areareduction67%.

The tempered steel was cold forged into /8" diameter hexagon headedbolts by a 78" bolt former (a three stage-transfer header) and thenscrewthreads were provided on the stems of the bolts while the boltswere being turned about the longitudinal axes thereof so as to producecomplete 118 8 mm. diameter (M X 35 mm.) hexagon headed bolts.

The formation of cold-forged bolts from a length of tempered steel wirerequires considerable mechanical working and specifically the steps ofshearing, rolling, heading, trimming, and thread-cutting of the temperedsteel wire. The tempered wire of Example II was subjected to these stepsduring the formation of the bolts and found to have, in spite of itshigh tensile strength, the same degree of cold-workability as that ofthe conventional annealed steel wire. Further, there were no breaks orcracks, flaws, etc., formed during the cold-forging operation.

The shear strength of bolts produced by the novel process was determinedthrough tension tests with wedges disposed at different angles such as10, 20 and 30, and the results of the tests are given below:

Tensile strength (kg/mm!) Location of breaks or cracks Bolt stem.

Do. Do.

Wedge angle (degrees) Bolts from Bolts by the tempered conventionalsteel Wire process Static test:

Yielding point (kg/mm?) 81. 5 8t. 5 Tensile strength (kg/mm?) 91. 0 95.5 Yield strength ratio 00. 0 91. 0 Fatigue test:

Zero-tension endurance limit (kg) 800.0 550. 0 Endurance limit, ultimatestrength ratio 140. 0 100. 0

Although there is no appreciable difference between the bolts by thenovel process and those by the conventional process in regard to fatigueresistance in static tests, it should be noted that in regard to fatigueresistance to repetitive stress the bolts from the tempered steel wirehave exceedingly high resistance as compared with that of the bolts bythe conventional process. For claim purposes sorbite defines theformation of martensite upon quenching from the austenizing temperatureand thereafter tempering at a temperature between 300 C. to 700 C.

It will be understood that modifications and changes may be made bythose skilled in the art without departing from the scope and spirit ofthe present invention. Accordingly, the invention is not to be limitedto the precise examples described hereinabove, but it is intended tocover also all changes, modifications and combinations of theembodiments described and/or claimed.

We claim:

1. A process for producing a cold-forged product which comprises thesteps of:

providing a length of metal wire from a continuously unwinding coil,said metal selected from the group consisting of carbon steel containingabout 0.2 to 0.6% and an alloy steel containing about 0.2 to 0.6%carbon, up to 3% manganese, up to 3% silicon and 0.0007 to 0.003% boronand at least one element selected from the group consisting of up to 3%chromium, up to 3% nickel and up to 3% molybdenum;

rapidly heating the Wire to a temperature above the A3 transition pointof said metal and within the range of 850 to 950 C. while the wire isrunning at a predetermined rate;

maintaining the metal within this temperature range for 1 to 3 minutesto sufficiently austenitize the same;

quenching the heated wire by means of a cooling agent until theinnermost portion of the interior of the wire is cooled to a temperatureof less than 200 C. thereby imparting a uniformly hardened martensitestructure to the wire;

rapidly reheating the cooled wire to a temperature of 300 to 700 C. forabout 2 to 10 minutes;

cooling the heated wire to about room temperature by means of a coolingagent so as to obtain a length of tempered wire having a uniform finegrain sorbite structure;

cold stretching the cold wire by an amount less than about 20% reductionin area; and

cold-forging the stretched wire into products having prescribed shapeand dimension by a cold-forging machine.

2. The process of claim 1 wherein said cooling agent 25 of the quenchingstep is selected from the group consisting of water and oil.

3. The process of claim 1 wherein the wire is cooled to effect temperingby being contacted with said secondmentioned cooling agent selected fromthe group consisting of water and air.

References Cited UNITED STATES PATENTS 1,924,099 8/1933 Bain et a1.148143 2,119,698 6/1938 Bayless 14812.4 2,121,415 6/1938 Vorn Braucke14812.4 2,341,264 2/1944 Coxe 14812.4 2,441,628 5/1948 Grifiiths et a1148143 2,527,731 10/1950 Ilacqua et a1. 148-12.4 3,053,703 9/1962 Breyer14812 3,235,413 2/1966 Grange et al. 14812.4

OTHER REFERENCES Pomp, A., The Manufacture and Properties of Steel Wire,1954, pp. 252-261.

L. DEWAYNE RUTLEDGE, Primary Examiner G. K. WHITE, Assistant ExaminerUS. Cl. X.R. 14812, 143

